This invention relates to a liquid crystal display device; and, more particularly, the invention relates to a high picture quality image active matrix system liquid crystal display device having thin film transistor elements.
A so-called transverse electric field system color liquid crystal display device is constructed such that transparent substrates are disposed in an opposed manner with a liquid crystal layer interposed therebetween, electrodes for display and reference electrodes are provided on liquid-crystal-side surfaces of regions of one or both of the transparent substrates which correspond to respective unit pixels, and electric fields are generated between these display electrodes and the reference electrodes parallel to the surfaces of the transparent substrates so as to modulate light which is transmitted through the liquid crystal layer. Such a color liquid crystal display device has been known to have a so-called excellent broad visual field angle which allows a person to recognize a clear image even from a position which is at a large angle relative to the display screen.
Liquid crystal display devices having such a construction are described in detail in, for example, Japanese publication of the translation of international patent application Hei 5-505247, Japanese patent publication Sho 63-21907 and Japanese laid-open patent publication Hei 6-160878.
However, a liquid crystal display element having the above-mentioned construction still has a problem in that an unnecessary electric field generated by the drain lines causes fluctuation of an electric field between the display electrodes and the reference electrodes, and so a bad image quality in which stripes are produced in a direction along the drain lines, or a so-called longitudinal smear (crosstalk), occurs. Means for solving this problem has been described in Japanese laid-open patent publication Hei 6-202127. The liquid crystal display element having the disclosed construction, however, is provided with shield electrodes and an electric potential is supplied from the outside; and, hence, it still has problems in that the charging or discharging of an electric current to a capacitance between the shield electrodes and the signal electrodes is large, the load on the drive circuit is large thus making the power consumption large and the drive circuit large, and a connecting means for applying the electric potential to the shield electrodes becomes necessary, thus increasing the steps and giving rise to connection failures.
The present invention has been made in view of the foregoing problems, and it is an object of the present invention to provide a liquid crystal display device which can suppress the occurrence of longitudinal smear and enhance the productivity, while also enabling a low power consumption.
To achieve the above object, the present invention is constituted by an active matrix system liquid crystal display device which includes a plurality of pixels which are composed of a plurality of drain lines and a plurality of scanning electrodes, and includes pixel electrodes and counter electrodes which are capable of applying an electric field parallel to substrate surfaces in the pixels, and image signals are capable of being supplied to the pixel electrodes from thin film transistors connected to the drain lines and gate lines, wherein the counter electrodes and the pixel electrodes are formed linearly so as not to overlap each other in a planar condition, and an insulating film having a specific dielectric constant not greater than 4 is formed on the drain line, and the counter electrodes are formed on the insulating film such that they cover the drain lines.
The invention is further constituted by an active matrix system liquid crystal display device, as described above, in which the pixel electrodes are formed on the insulating film.
The is further constituted by an active matrix system liquid crystal display device, as described above, in which the insulating film and at least either one of a gate insulating film and a passivation layer of the thin film transistor element are formed in the same pattern.
The invention is further constituted by an active matrix system liquid crystal display device, as described above, having light shielding films extending horizontally in stripes.
The invention is constituted by an active matrix system liquid crystal display device, as described above, which has an insulating film having a film thickness of not smaller than 1 xcexcm and not greater than 3 xcexcm.
The invention is constituted by an active matrix system liquid crystal display device, as described above, which an the insulating film made of a resist material.
The invention is constituted by an active matrix system liquid crystal display device, as described above, in which an inorganic insulating film which protects the thin film transistor element has a film thickness of not smaller than 0.05 xcexcm and not greater than 0.3 xcexcm.
The liquid crystal display element having the above-mentioned construction is produced from the following three operations.
Reference electrodes are formed on an organic insulating film in such a manner that the reference electrodes are completely overlapped on drain lines formed on one transparent substrate side in a plan view; and, hence, almost all of the unnecessary lines of electric force which are generated by the drain lines are terminated at the reference electrodes. Ac6ordingly, the crosstalk due to the leakage electric field which is peculiar to a display system, such as the display system of the present invention which adopts a transverse electric field, can be resolved. In this manner, the leakage electric field is more completely shielded than it would be by the shield electrodes which have been conventionally disposed at both sides of the drain line or on the counter substrate; and, hence, the horizontal direction of the pixels can be occupied by the display electrodes, reference electrodes and opening portions. Furthermore, it is also unnecessary to hide the gap between the drain line and the reference electrode, and, hence, a light insulating film (black matrix) in a vertical direction can be eliminated. Therefore, a low aperture efficiency, which is the largest defect in the display system adopting a transverse electric field, can be drastically improved and an aperture efficiency exceeding 50% can be realized. Namely, according to the present invention, a high aperture efficiency and a low smear condition are compatible.
The specific dielectric constant of the organic insulating film is approximately half (the specific dielectric constant xcex5r being approximately 3) that of the inorganic insulating film. Furthermore, since the thickness of the organic passivation layer can be easily increased compared to the inorganic passivation layer, the distance between the drain line and the reference electrode is expanded. Even when these drain lines are entirely covered with the reference electrodes, the capacity formed between the drain lines and the reference electrodes can be made considerably small. Accordingly, the load as seen from the drain lines is reduced, so that the wiring propagation delay of the image signal becomes small, and the signal voltage is sufficiently charged into the display electrodes and the drive circuit for driving the drain lines can be minimized.
The organic passivation layer has an excellent flatness so that when the organic passivation layer is coated on the uppermost layer of the substrate which constitutes an active element, the flatness of the substrate which constitutes the active element is enhanced. Accordingly, the irregularities of the brightness (transmission factor)-voltage characteristics caused by the irregularities of the gap between the substrates can be eliminated thus enhancing the uniformity of the brightness.